This strip uses a play on the homophonic relationship between "canon", the literary term, and "cannon", a projectile weapon. The word headcannon is a compound of "head" and "cannon".

In this strip, Black Hat tells Cueball that he has a "new headcannon". Cueball, thinking Black Hat means "headcanon" (an unjustified belief or theory about a fictional universe), inquires what Black Hat's new idea is. Instead of the expected idea or theory, Black Hat removes his hat to reveal a tiny cannon on his head which blows away Cueball and his computer desk.

While headcanon may often be ignored or dismissed as a personal theory, a headcannon would be far harder to ignore, as it is a physical object which has a notable (and in this case violent) impact on the real world.

In the title text Randall makes the spellings of these two words indistinguishable by using three consecutive "n"s to spell "headcannnon". Therefore, the title text is deliberately vague. It could be interpreted that it is easy to convince people that you have a cannon on your head, that it is easy to make people believe in a self invented headcanons, or both. Since you are choosing your own interpretation of this title text, the joke is that you are creating your own headcanon.

This comic also shows Cueball being once again distracted from his work in a manner similar to 1388: Subduction License.

In terms of a given literary series, "canon" describes a set of works that are collectively recognized by the community as having authenticity. Generally, works created or endorsed by the original author(s) are considered canonical. Not all original content is considered canon and not all canon is original content. Sometimes creators will rewrite the canon (called a retcon) and make things that were previously canonical non-canonical. For example, the origins of a character may be rewritten, thus invalidating the portions of the works that speak to the old origins. Other times creators will incorporate non-original content and therefore incorporate the canon of these borrowed works.

A headcanon as the name implies is a form of canon that only exists in one's mind. More specifically, a headcanon is created when a consumer watching or reading the material develops their own ideas about a fictional universe that are not actually part of the canon, perhaps developing their own backstories or experiences for characters. Some frequent examples of headcanon include relationships between characters, abilities, events following the conclusion of the work, etc. which the author or creator has not explained or included. For example, a consumer may "read between the lines" and assume that there was a previous romantic relationship between two characters where no conclusive evidence actually exists of one. Some fans who come up with particularly interesting or convincing headcanons may decide to share them with others in hopes that their idea spreads.

Discussion

Why are there three n's in headcannnon in the title text?
Keavon (talk)

Or as n increases the effort to convince others that the existence/correctness of headca(n)+on decreases? 108.162.216.26 20:31, 30 July 2014 (UTC)arcturius

I think it's as simple as 1 n in canon (what the pun is based on), 2 n's in cannon (in the comic), and just to keep the pattern going, 3 n's in cannnon (in the title text).--173.245.54.175 05:35, 30 July 2014 (UTC)

That reminds me on Neil Stephensons - The Diamond Age: or A Young Lady's Illustrated Primer... Very nerdy! 108.162.254.21 (talk) (please sign your comments with ~~~~)

Another very common usage of headcanon is when you REMOVE something from your headcanon - that is, pretend that it never happened, despite it being canon. Often it's case of not-really-good sequels. Or later edits: see Han shot first. -- Hkmaly (talk) 10:35, 30 July 2014 (UTC)

I thought that headcanon was everything fans imagined, not just what contradicts canon. --141.101.105.204 16:32, 30 July 2014 (UTC)

Anyone note that the computer is completely undamaged (from the cannonfire at least, no telling about when it strikes the floor), despite the desk being demolished? Zowayix (talk) 13:14, 30 July 2014 (UTC)

Black Hat is shown to have short dark hair. That's new xkcd canon. As far as I know, he'd always been shown wearing a hat completely covering his hair until now. --Dangerkeith3000 (talk) 15:33, 30 July 2014 (UTC)

"The title text is a pun on the homophones *canon* and *cannon*" ... uh, the whole entire COMIC is a pun on the homophones *canon* and *cannon*. 173.245.56.149 18:16, 30 July 2014 (UTC)

Kudos to the author(s) of the example using Quark. One of the best-written explanations on this wiki. jameslucas(" " / +) 22:42, 30 July 2014 (UTC)

"Fans might wonder why, on a station that has "replicators" (devices that can create any food or drink out of energy on demand), anyone would patronize a bar" - perhaps because they might want to, you know, socialise with other people? Call me old-fashioned... --141.101.99.37 14:23, 31 July 2014 (UTC)

Anyone know if this weeks what if is different depending on region? I only ask because it mentions my small town and I am skeptical based on past comics. 1037: Umwelt173.245.56.208 06:25, 1 August 2014 (UTC)

No one has explained why the comic is titled "New". Because Black hat says "NEW HEADCANNON:" rather than "I HAVE A NEW HEADCANNON:", I think he is speaking not English, but some programming language. Black Hat created the headcannon by saying "new Headcannon:", which is a command to instantiate an object of type Headcannon. This is similar to previous strips http://xkcd.com/353/ and http://xkcd.com/413/, which attributed supernatural creative powers to Python's "import" statement. But "new Headcannon:" isn't Python. I don't know language it is. 108.162.237.178 (talk) (please sign your comments with ~~~~)

Best I can come up with is a Quick BASIC label, but if that were the case instantiating it would have required a precedant gosub, not new. Sailorleo (talk) 21:12, 22 October 2015 (UTC)

It's English; he's just speaking in an informal context, and so is using the sort of colloquialisms that don't work in more formal registers and which look weird when written. "New headcannon" (or, indeed, "New headcanon") is just shorthand for "I have acquired/developed/accepted/stolen a new headcann?on". Hppavilion1 (talk) 23:45, 21 December 2016 (UTC)

Could the "headcannnon" refer to an idea that "blows your mind"? The trajectory of the "literal" cannon ball ends where Cueball's head was, so it went from head to head, not head to desk... --B. P. (talk) 22:34, 28 October 2014 (UTC)

It seems more likely Black Hat is just being an asshole and shooting him in the face. -Pennpenn 108.162.249.205 02:51, 23 January 2015 (UTC)

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